Exclusive - Virgin's Branson invests in payment start-up Square

<p>Virgin Group head Sir Richard Branson announces plans to launch a solo piloted submarine to the deepest points of the world's five oceans at a news conference in Newport Beach, California April 5, 2011.Alex Gallardo</p>

SAN FRANCISCO (Reuters) - Square, the mobile payment start-up that counts former U.S. Treasury Secretary Lawrence Summers on its board of directors, has a new big-name backer: British billionaire Richard Branson.

Branson has made a "multimillion" dollar investment in Square, according to a Square spokeswoman who declined to disclose the exact amount.

Branson's investment in Square follows a $100 million (62 million pounds) investment in June led by venture capital firm Kleiner Perkins Caufield & Byers and an investment for an undisclosed amount by credit card company Visa in April.

Square's technology lets merchants accept payments using tablet PCs or mobile phones. The company recorded $10 million in daily transactions for the first time this past weekend, spokeswoman Katie Baynes told Reuters, up from the $4 million per day in gross payment volume that it announced during the summer.

Square has said it is on track to process $2 billion worth of transactions on an annual basis.

The two-year-old company, led by Twitter co-founder Jack Dorsey, faces deep-pocketed competitors eager for a slice of the nascent mobile payment business. Search giant Google Inc (GOOG.O) recently introduced a digital wallet that lets people use their smartphones to pay for clothing, food and other items.

Branson's investment in Square is a "personal investment," said Baynes.

Branson is the founder and highly public face of the Virgin Group, which has created more than 300 businesses that range from air travel to mobile phones.